The claimed invention relates generally to the field of disc drive data storage devices and more particularly, but not by way of limitation, to a method and apparatus for accelerating a disc drive spindle motor from rest to a final operational velocity.
A disc drive is a data storage device used to store digital data. A typical disc drive includes a number of rotatable magnetic recording discs which are axially aligned and mounted to a spindle motor for rotation at a high constant velocity. A corresponding array of read/write heads access tracks defined on the respective disc surfaces to write data to and to read data from the discs.
Disc drive spindle motors are typically provided with a three-phase, direct current (dc) brushless motor configuration. The phase windings are arranged about a stationary stator on a number of radially distributed poles. A rotatable spindle motor hub is provided with a number of circumferentially extending permanent magnets in close proximity to the poles. Application of current to the windings induces electromagnetic fields which interact with the magnetic fields of the magnets to apply torque to the spindle motor hub and induce rotation of the discs.
Due to the prevalence of numerous consumer devices that employ electrical motors, it might seem at first glance that accelerating a disc drive spindle motor from rest to a final operational velocity would be relatively straightforward; simply turn on the motor and let it accelerate to the final desired speed. As those skilled in the art will appreciate, however, accelerating a spindle motor from rest can be relatively difficult and involves a number of important considerations that must be adequately taken into account.
First, it is important to accurately determine the rotational state of a disc drive spindle motor prior to application of drive signals to the motor. Application of drive signals to a spindle motor while the motor is in an unknown state could lead to the inadvertent rotation of the motor in the wrong direction. Rotating the spindle motor in the wrong direction, even for a very short time, can lead to premature failure of a disc drive; heads and disc surfaces can be damaged, and lubricating fluid used in hydrodynamic spindle motor bearings can be pumped out of the bearings.
Early disc drive spindle motor designs used Hall effect or similar external sensors to provide an independent indication of motor positional orientation. However, present designs avoid such external sensors and instead use electronic commutation and back electromagnetic force (bemf) detection circuitry to provide closed-loop spindle motor control, such as discussed in U.S. Pat. No. 5,631,999 issued to Dinsmore. Such approach generally entails applying a predetermined sequence of commutation steps to the phase windings of the spindle motor over each electrical revolution (period) of the motor. A commutation step involves supplying the motor with current to one phase, sinking current from another phase, and holding a third phase at a high impedance in an unenergized state.
Detection circuitry measures the bemf generated on the unenergized phase, compares this voltage to the voltage at a center tap of the windings, and outputs a signal at a zero crossing of the voltages; that is, when the bemf voltage changes polarity with respect to the voltage at the center tap. The point at which the zero crossing occurs is then used as a reference for the timing of the next commutation pulse, as well as a reference to indicate the position and relative speed of the motor.
Above an intermediate operational speed, the control circuitry will generally be able to reliably detect the bemf from rotation of the spindle motor, and will further be able to use the detected bemf to accelerate the motor to a final operational velocity. Below this intermediate speed, however, closed-loop motor speed control using detected bemf generally cannot be used since the spindle motor will not generate sufficient bemf at such lower speeds. Thus, other methodologies are required to detect the rotational position of the spindle motor when the motor is at rest.
Once the initial state of the motor is determined, a related consideration is getting the motor to properly and safely rotate up to the intermediate velocity so that the closed-loop motor control circuitry can take over and accelerate the motor up to the operational velocity. This can be accomplished, for example, by applying short duration pulses followed by position measurements to detect each successive commutation state transition. The velocity of the motor can be measured in relation to the elapsed time between successive commutation state transitions.
Further considerations are encountered once the motor reaches the intermediate velocity and transition to closed-loop acceleration takes place. Disc drives are typically installed as peripheral devices in larger data management systems (e.g. computer systems). The power consumption of a drive will typically be limited by the available power supply resources of the system.
It is common to invoke maximum current consumption specifications (both instantaneous peak current and root-mean-square (RMS) current) upon a drive design. Such specifications limit the rate at which current can be applied to a spindle motor during closed-loop acceleration. At the same time, customer requirements continue to demand shorter and shorter overall initialization times for drives to go from a deactivated (off) state to an operationally ready state. The time required to accelerate a spindle motor to the final operational velocity can comprise a significant portion of the total initialization time.
There is a need for improvements in the art to enable a spindle motor to accelerate from rest to a final operational velocity in a fast and reliable manner while maintaining power consumption requirements within specified levels. It is to such improvements that the present invention is directed.
In accordance with preferred embodiments, a disc drive includes a brushless direct current (dc) spindle motor which rotates a disc at an operational velocity during data transfer operations between the disc recording surface and a host device. The disc drive electrically commutates the spindle motor in response to detected back electromotive force (bemf) from the spindle motor during rotation.
The spindle motor is initially accelerated from rest to an intermediate velocity at which sufficient bemf is generated by the rotation of the spindle motor to enable commutation circuitry of the disc drive to time the application of drive pulses to the spindle motor. The spindle motor is then accelerated from the intermediate velocity to the operational velocity by applying a velocity dependent reference profile which establishes a sequence of reference levels to control the flow of current through the spindle motor during acceleration. The sequence of reference levels have different magnitudes at different velocities of the spindle motor between the intermediate velocity and the operational velocity.
Preferably, the sequence of reference levels comprise reference voltages which are compared to the voltage at a node of the spindle motor to generate a comparison signal which controls the flow of current through the spindle motor. In preferred embodiments, the velocity dependent reference profile comprises a pulse width modulated (PWM) signal having different duty cycles at the different velocities of the spindle motor during acceleration; in other preferred embodiments, the velocity dependent reference profile comprises a sequence of digital values having different magnitudes at the different velocities of the spindle motor during acceleration.
Preferably, a current limit specification threshold is provided indicative of a maximum acceptable level of current that can flow through the spindle motor as the spindle motor is accelerated from the intermediate velocity to the operational velocity. The velocity dependent reference profile is selected to maintain the magnitude of the current that flows through the spindle motor during acceleration below the specification threshold.
The velocity dependent reference profile is preferably selected by using an initial reference profile to accelerate the spindle motor from the intermediate velocity to the operational velocity. The current that flows through the spindle motor is measured during such acceleration, and the velocity dependent reference profile is selected in relation to the measured current. In this way, higher reference levels can be utilized at times during the acceleration period when margin is available to accelerate the motor faster without exceeding the specification threshold.
These and various other features and advantages which characterize preferred embodiments of the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.